Unpinned oxide-compound semiconductor structures and method of forming same

ABSTRACT

Unpinned epitaxial metal-oxide-compound semiconductor structures are disclosed and a method of fabricating such structures is described. Epitaxial layers of compound semiconductor are grown by MBE which result in the formation of a smooth surface having a stabilized reconstruction. An elemental semiconductor layer is deposited epitaxially in situ with the compound semiconductor layer which unpins the surface Fermi level. A layer of insulator material is then deposited on the elemental semiconductor layer by PECVD. In one embodiment, the compound semiconductor is GaAs and the elemental semiconductor is Si. The insulator material is a layer of high quality SiO 2 . A metal gate is deposited on the SiO 2  layer to form an MOS device. The epitaxial GaAs layer has a density of states which permits the interface Fermi level to be moved through the entire forbidden energy gap. In another embodiment, the SiO 2  deposition completely consumes the interface Si layer so that the resulting MOS device comprises SiO 2  directly overlying the GaAs layer.

This is a divisional of copending application Ser. No. 207,700, filed on June 15, 1988, now U.S. Pat. No. 4,987,095 issued Jan. 22, 1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to metal oxide semiconductor (MOS) structures and more particularly to MOS structures on a compound semiconductor, such as GaAs, and to methods of forming a gate insulator, such as SiO₂, on a compound semiconductor.

2. Description of the Prior Art

Heretofore, attempts to realize metal-oxide-semiconductor structures on GaAs have been unsuccessful due to the very high density of interface states which are nearly always present at exposed GaAs surfaces or at interfaces with virtually all materials. The presence of the high density of interface states causes the pinning of the GaAs interface Fermi level. The pinning of the interface Fermi level is so tenacious, that it is the basis for reproducible metal-semiconductor GaAs MESFET devices. The Fermi level is pinned approximately midgap, i.e. midway between the conduction band energy and the valence band energy at the surface.

It is desirable, however, in many applications to unpin the Fermi level. Prior art attempts to unpin the Fermi level at the surface have involved chemical treatments of the GaAs surface. These include the formation of surface Ga oxides by treatment with water and light as described by Offsey, et al. Appl. Phys. Lett. 48, 475 (1986) and passivation by a sodium sulfide treatment, as describe Sandroff, et al., Appl. Phys. Let. 51, 33 (1987). In addition, it is known that thin overlayers of Ge or Si on GaAs can result in different barrier heights, showing modification of surface Fermi level position, as shown by Grant, et al., J. Vac. Sci. Tech. B5(4) 1015 (1987). It has not been shown in any prior work, however, that the surface Fermi level position can be moved throughout the forbidden energy gap.

In the fabrication of MOS structures on GaAs, the surface Fermi level should be unpinned to allow the Fermi level to be varied by bias voltages applied to the gate electrode. The movement of the Fermi level permits the device to be operated in an inversion mode or an accumulation mode, depending on the polarity of the bias applied to the gate. In addition, a low leakage gate insulator material is necessary to maximize the gate control of charge carriers.

None of the prior art unpinning techniques have allowed the formation of a low leakage insulating material on the treated surface, without repinning the interfacial Fermi level. MOS-capacitor structures with a leaky dielectric generally exhibit poor C-V characteristics, since the channel and metal layers can exchange charge. Even a GaAs surface which is unpinned will not exhibit good charge control by the gate electrode when the insulator is leaky.

SUMMARY OF THE INVENTION

The present invention is directed to oxide-compound semiconductor structures and a method of fabrication which provides a structure having both an unpinned compound semiconductor surface as well as a low leakage gate insulator material. The gate leakage current is minimized by the incorporation of a high quality plasma enhanced chemical vapor deposition (PECVD) oxide. The leakage current and interface state density are low enough to allow the interfacial Fermi level to be swept through the entire semiconductor bandgap, i.e. from accumulation to inversion, corresponding to an unpinned compound semiconductor surface. The present invention may be implemented with compound semiconductors of the III-V system as well as the II-VI system materials. The invention will be described hereinafter by reference to a GaAs compound semiconductor material, however, the invention is not limited to any particular III-V compound material.

Generally, the structure of the invention comprises of a GaAs substrate, various III-V epitaxial layers having a thickness, doping and composition dictated by the particular device for which the structure is designed, an interfacial elemental semiconductor layer and an insulating layer of semiconductor material. A gate metal which is determined by processing issues is disposed on the insulator to form the basic MOS device. The elemental semiconductor material may be selected from a group consisting of silicon and germanium and the insulating layer comprises an oxide of silicon. As with the compound semiconductor, the invention will be described by way of example only, with the interfacial layer being made of elemental silicon and the insulating layer comprising SiO₂.

In the method of fabrication, (100) GaAs epitaxial layers are grown under conditions which produce a smooth surface with a stabilized reconstruction. The reconstruction may take many forms such as, for example, a As-stabilized c(2×8) reconstruction. Growth by molecular beam epitaxy (MBE) generally provides smoother and better ordered surfaces than other methods. An epitaxial layer of elemental Si is then formed .on the epitaxial GaAs. The Si layer is deposited in situ with the GaAs epitaxial layer. Thereafter, a layer of insulator material is deposited on the Si semiconductor layer and a metal gate is deposited on the insulator layer.

The thickness of the Si layer is sufficient to prevent the insulator material from being introduced into the GaAs epitaxial layer to prevent oxidation of the GaAs. After the Si layer is deposited, the structure is removed from the MBE system and then loaded into another system for the deposition of the SiO₂. During this time, a very thin native oxide layer is formed on the Si. The SiO₂ layer is deposited using a PECVD technique that provides a high quality oxide under low temperatures. The structure is then heat treated and the gate metal is deposited by conventional techniques to form an MOS-device. Alternatively, the SiO₂ layer may be deposited in situ with the deposit of the GaAs and Si layers by transferring the Si/GaAs structure to an adjacent CVD chamber without breaking vacuum.

The structure of the invention comprises an epitaxially grown compound semiconductor GaAs layer having a density of states which permits the interface Fermi level to be moved through the entire forbidden energy gap in the presence of an electric field. A layer of SiO₂ insulator material of low electrical leakage overlies the compound semiconductor material and an epitaxial, single crystal, continuous layer of elemental Si semiconductor is disposed between the compound semiconductor layer and the layer of insulating material. A metal gate is provided on the insulator material to provide a basic MOS device such as a capacitor. Source and drain regions may be formed by conventional ion implantation or other techniques in the compound semiconductor layer and separated by the gate to form a MOSFET. Thus, the present invention provides a GaAs MOSFET which retains the superior charge control characteristics of the SiO₂ /Si system as well as the superior transport properties of GaAs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram of the GaAs MOS structure of the present invention.

FIG. 2 is a schematic cross-sectional diagram of another embodiment of the GaAs MOS structure of the invention.

FIGS. 3 and 4 are graphs showing the capacitance versus voltage and the current versus voltage characteristics respectively of a p-type MOS capacitor formed in accordance with the present invention.

FIG. 5 is a schematic cross-sectional diagram of a MOSFET of the present invention.

FIG. 6 is a graph showing the source-drain characteristics of a MOSFET of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the structure 10 of the invention comprises a GaAs substrate 12 upon which a plurality of (100) GaAs epitaxial layers 14 are grown by molecular beam epitaxy under growth conditions which produce a smooth surface, with a surface roughness of three monolayers or less. While the MBE technique is preferred for its ability to provide highly ordered smooth surfaces, the epitaxial growth may also be performed in a chemical vapor deposition system. The epitaxial growth is typically performed in an ultra high vacuum environment. The growth of the GaAs is terminated under conditions which result in the formation of a stabilized reconstruction at the surface. Typically, GaAs epitaxial growth by MBE results in an As-stabilized c(2×8) reconstruction. Other III-V materials may also result in anion stabilized reconstruction on the surface. For example, in an InP system, a P-stabilized reconstruction will be provided. Metal rich, such as Ga, stabilized reconstructions may also be present. Examples of typical reconstructions are c(4×4), c(2×8), c(8×2), 1×6 and 4×6, which are listed in the order of increasing Ga surface concentration. These reconstructions may be obtained under various conditions as shown, for example, by Bachrach, et al., J. of Vac. Sci. Tech. B5(4), 1135 July/August 1987.

When a As-stabilized c(2×8) reconstruction is provided, the background As vapor is then reduced by shuttering and/or cooling the As source. The substrate is then cooled to a temperature in the range of between 300°-400° C., in such a way as to maintain the As c(2×8) reconstruction. The surface stoichiometry is directly related to the surface reconstruction, and it is expected that the surface stoichiometry will have a profound influence on the electrical properties of the structure.

A Si layer 16 is then deposited onto the clean surface of layers 14 in situ, i.e. in the same ultra high vacuum environment MBE or CVD system used to grow the epitaxial layers 14. The Si layer 16 is deposited at a low rate, for example, approximately 0.1 angstroms per second. The Si layer thickness is determined by several constraints. The minimum thickness is determined by the minimum amount of Si which is necessary to cover and unpin the GaAs surface. In addition, the thickness must be sufficient to prevent the subsequent SiO₂ deposition from penetrating and oxidizing the GaAs. The maximum thickness of the Si layer is limited by the thickness which does not allow significant electrical conduction in Si. It is desirable to confine the electrical carriers in a channel in the GaAs layer since the transport properties of GaAs are superior to Si. The Si layer 16 may be between 5 and 100 angstroms thick. Typically, the thickness will be between 10 and 40 angstroms.

Under the deposition conditions described above, the Si adatoms have enough time to allow the formation of an epitaxial layer, without a significant amount of intermixing with the substrate. Reflection electron diffraction patterns observed during the growth of the Si layer indicate that the substrate c(2×8) reconstruction is removed after approximately 5 angstroms of Si have been deposited, and that the Si layer is single crystal. At layer thicknesses in the range of tens of angstroms, it is likely that the Si layer is also pseudomorphic, i.e. is strained to have the same lattice constant as the substrate. The cause of the unpinning of the GaAs surface by the Si is not completely understood. It is believed that the smooth, reconstructed GaAs surface is essential in controlling the Si/GaAs bonding and forming an abrupt low-defect density heterointerface. The GaAs dangling bonds which normally cause Fermi level pinning are satisfied, thereby removing the interface states from the forbidden gap.

After the Si layer 16 is deposited, the structure is removed from the MBE system and then loaded into another system for the deposition of an SiO₂ layer 18. During this time, the structure surface is exposed to air. It is expected that the Si surface layer forms a very thin, yet stable oxide and that the Si and native oxide layers form a protective layer on the GaAs surface. The SiO₂ layer 18 is preferably deposited by a PECVD technique which produces a high quality oxide utilizing low substrate temperatures during deposition. The SiO₂ layer is deposited at a rate of approximately 60 angstroms per minute at a substrate temperature of approximately 350° C., to a thickness of 30 to 500 angstroms. The quality of the PECVD oxide is known to be sensitive to the contamination on the starting substrate surface as well as the surface roughness. Both of these factors are minimized for MBE layers which are grown under UHV conditions and which have extremely smooth surfaces. The preferred PECVD technique is disclosed in more detail in the following articles by Batey et al., both of which are incorporated herein by reference:. Low Temperature Deposition of High Quality Silicon Dioxide by Plasma Enhanced Chemical Vapor Deposition, J. of Appl. Phys., 60(9), 3136, 1 Nov. 1986, and Electrical Characteristics of Very Thin SiO₂ Deposited at Low Substrate Temperatures, IEEE Elec. Dev. Lett., Vol. EDL-8, No. 4, 148, Apr. 1987.

In an alternative embodiment, all the layers of the structure are deposited in situ. In this method, the Si/GaAs structure is transferred to a CVD chamber without breaking the vacuum present during deposition of the epilayers. The SiO₂ insulator layer is thereafter deposited by a CVD process. In this embodiment, the formation of a native oxide is avoided. The in situ deposition of all the layers is much cleaner as there is no exposure to contaminates present in the ambient.

In the deposition of the SiO₂, at least initially, a certain amount of the oxide penetrates into the elemental silicon layer. As stated above, the thickness of the silicon layer 16 must be sufficient so that the SiO₂ deposition does not penetrate and oxidize the epitaxial GaAs layer 14. Typically, the Si layer 16 will be sufficiently thick so that after deposition of the SiO₂ layer, a thin layer of Si will remain in the final structure. However, the thickness of the Si layer 16 can be grown so that all of layer 16 is consumed by the SiO₂ deposition. Thus, the present invention can provide a final structure having only the SiO₂ layer 18 disposed on the GaAs epitaxial layer 14. Heterostructure 11 in FIG. 2 is an example of such a final structure. Hence, in this embodiment, the thin Si layer 16 inhibits oxidation of the GaAs while allowing the full consumption of layer 16 so that the SiO₂ insulator layer 18 is in direct contact with the GaAs epitaxial layer 14.

The layers at this point are heated to 400°-750° C. in various inert gas ambients for times in the range of 15 minutes and longer. The gate metal is then deposited by conventional techniques, and the structures are subjected to additional heat treatment. The heat treatments are very similar to that employed in standard silicon MOS technology, as is the photolithographic processing. Heat treatment is necessary to reduce the interface state density.

The electrical characteristics of heat treated MOS capacitors are shown in FIGS. 3 and 4. The capacitance versus voltage (C-V) characteristics shown in FIG. 3 clearly show that an inversion layer can be formed in the epitaxial GaAs layer 14, and that the interfacial Fermi level can be moved through the entire bandgap. The capacitor of FIG. 3 was fabricated with a gate metal comprised of tungsten silicide. The sample was given a pre-deposition anneal of 650° C. in nitrogen for 30 minutes, blanket WSi was deposited, and a subtractive etch was performed to define the gate. A post-metal anneal at 450° C. for 30 minutes in forming gas completed the device. The quasi-static C-V curve a of FIG. 3 was swept at 0.1 V/s and the high frequency (1MHz) curves b and c were swept at 0.5 V/s in the directions indicated by the arrows. The gate capacitance in FIG. 3 is normalized for C_(max) which is equal to the oxide capacitance C_(ox), which for this sample is equal to 45 pF. The hysteresis, stretch out, and large difference between the high and low frequency C-V curves indicate both the presence of slow and fast states near the interface. The current versus voltage characteristics shown in FIG. 4 show nearly classic charge injection behavior from the gate metal, which for this sample was Al, into the oxide, and is indicative of a very high quality oxide and metal/oxide interface. Curve d is for a negative bias applied to the gate and curve e is for a positive bias. A GaAs MOSFET 13 manufactured in accordance with the invention is shown schematically in FIG. 5. After fabrication of layers 12, 14 and 16 as shown in FIG. 1, the gate and gate insulator regions 20 and 18 are defined by typical processing techniques and source and drain regions 22 and 24 are formed, for example, by ion implantation. The GaAs MOSFET characteristics of I_(D) versus V_(DS) for V_(G) applied in one volt steps are shown in FIG. 6, in which the MOSFET was provided with a tungsten silicide gate. The characteristics shown in FIG. 6 are for a device operating in the depletion mode, with a transconductance of 30mS/mm and a gate length of 8 μm. Transconductances have been obtained up to 60 mS/mm in a device having complete pinch off.

While the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention. 

Having thus described our invention, what we claim as new, and desire to secure by letters patent is:
 1. A semiconductor structure comprising:an epitaxially grown compound semiconductor layer having a forbidden energy gap and an interface Fermi level, said compound semiconductor layer having a density of states which permits the interface Fermi level to be moved through the entire forbidden energy gap in the presence of an electric field; a layer of insulator material of low electrical leakage overlying said compound semiconductor layer having a thickness sufficient to act, as a gate insulator in a field effect transistor environment; and an epitaxial, single crystal, continuous layer of elemental semiconductor disposed between said compound semiconductor layer and said layer of insulating material.
 2. The semiconductor structure according to claim 1 further including a conductive element disposed on the surface of said layer of insulating material.
 3. The semiconductor structure according to claim 1 further including at least a substrate of a compound semiconductor material disposed beneath said compound semiconductor layer.
 4. The semiconductor structure according to claim 1 wherein said layer of elemental semiconductor has a thickness sufficient to prevent introduction of said insulator material into said compound semiconductor.
 5. The semiconductor structure according to claim 1 wherein said compound semiconductor is a binary, ternary or quaternary compound of elements of Groups III and V of the periodic table of the elements and alloys thereof.
 6. The semiconductor structure according to claim 5 wherein said compound semiconductor layer is gallium arsenide.
 7. The semiconductor structure according to claim 1 wherein said elemental semiconductor layer is silicon.
 8. The semiconductor structure according to claim 1 wherein said layer of insulator is silicon dioxide.
 9. The semiconductor structure according to claim 1 wherein said compound semiconductor layer has a (100) crystallographic orientation.
 10. A semiconductor structure comprising:an epitaxially grown GaAs layer having a forbidden energy gap and an interface Fermi level, said GaAs layer having a density of states which permits the interface Fermi level to be moved through the entire forbidden energy gap in the presence of an electric field; a layer of SiO₂ insulator material of low electrical leakage overyling said GaAs layer having a thickness sufficient to act, as a gate insulator in a field effect transistor environment, and an epitaxial, single crystal, continuous layer of Si disposed between said GaAs layer and said SiO₂ layer from said GaAs layer.
 11. A semiconductor structure according to claim 10 further including a metal gate disposed on the surface of said SiO₂ layer.
 12. A semiconductor structure according to claim 11 further including at least a substrate of GaAs disposed beneath said epitaxially grown GaAs layer.
 13. A semiconductor structure comprising:an epitaxially grown semiconductor layer having a forbidden energy gap and an interface Fermi level, said compound semiconductor layer having a density of states which permits the interface Fermi level to be moved through the entire forbidden energy gap in the presence of an electric field; and a layer of insulator material of low electrical leakage overlying and in direct interface with said compound semiconductor material.
 14. The semiconductor of claim 13 wherein the compound semiconductor layer has an unpinned highly ordered smooth surface.
 15. The semiconductor structure of claim 14 further including a conductive element disposed on the surface of said layer of insulating material.
 16. The semiconductor structure according to claim 15 further including at least a substrate of a compound semiconductor material disposed beneath said epitaxially grown compound semiconductor layer.
 17. The semiconductor structure of claim 16 wherein said compound semiconductor is a binary, ternary or quarternary compound of elements of groups III and V of the periodic table of elements and alloys thereof.
 18. The semiconductor structure of claim 17 wherein the compound semiconductor layer is gallium arsenide.
 19. The semiconductor of claim 18 wherein said insulator layer is silicon dioxide.
 20. The semiconductor structure of claim 19 wherein the compound semiconductor layer as a (100) crystalographic orientation.
 21. The semiconductor structure of claim 15 further including source and drain regions formed within said compound semiconductor and spaced from each other on opposites of said insulator material.
 22. A field effect transistor comprising:an epitaxially grown compound semiconductor layer having a forbidden energy gap and an interface Fermi level, said compound semiconductor layer having a density of states which permits the interface Fermi level to be moved through the entire forbidden energy gap in the presence of an electric field; an epitaxial, single crystal, continuous layer of elemental semiconductor disposed on said compound semiconductor layer; a gate insulator layer of insulator material of low electrical leakage overlying said layer of elemental semiconductor; a conductive element disposed on the surface of said insulating layer; and source and drain regions disposed within said layer of compound semiconductor and spaced from each other on opposite sides of said gate insulator layer.
 23. The field effect transistor of claim 22 wherein said compound semiconductor is a binary, ternary or quarternary compound of elements of groups III and V of the periodic table of elements and alloys thereof.
 24. The field effect transistor of claim 23 wherein said compound semiconductor layer is gallium arsenide.
 25. The field effect transistor of claim 24 wherein the elemental semiconductor layer is silicon.
 26. The field effect transistor of claim 25 wherein the layer of insulator is silicon dioxide.
 27. The field effect transistor of claim 26 wherein the compound semiconductor layer has a (100) crystal graphic orientation.
 28. The semiconductor structure according to claim 1 wherein said compound semiconductor layer has an unpinned highly ordered smooth surface.
 29. A semiconductor structure according to claim 10 wherein said GaAs layer has an unpinned highly ordered smooth surface.
 30. The field effect transistor of claim 22 wherein said compound semiconductor layer has an unpinned highly ordered smooth surface. 